Red mud dewatering and washing process

ABSTRACT

The invention covers a process for dehydration/filtration, and washing of red mud, if applicable with addition of sand, particularly from the Bayer process for aluminum production from bauxite, in which filters are used and caustic is washed out on the filter. The invention is basically characterized by washing and dehydration being effected under pressure above atmospheric with regeneration of bound caustic and production of a residue with a high solids concentration. In addition, the invention covers a plant for the implementation of the process.

FIELD OF THE INVENTION

The invention covers a process for filtration and washing of red mud, ifapplicable with addition of sand, in particular from the Bayer processfor aluminum production from bauxite, in which filters are used andcaustic is washed out on the filter.

BACKGROUND OF THE INVENTION

Alumina (aluminum oxide Al₂ O₃) is generally obtained by causticleaching of bauxite, the so-called Bayer process, as an intermediateproduct in the electrolytic production of aluminum. Red mud, the residueof the Bayer process, must be disposed of in considerable quantities.Red mud removal is one of the major technical and economic problems ofthe Bayer process. Most of the timer the closed circuit is operated witha supersaturated alumina solution at elevated caustic concentrations andtemperatures. The solid and finely divided red mud must be separatedfrom this circuit and washed out. During this process, concentration andtemperature of the liquor are reduced, producing undesired sedimentationof alumina. This alumina, as well as the liquid containing caustic andadditional quantities of alumina disposed of together with the red mud,account for a considerable loss which, however, can be reduced bysuitable process parameter control. The lower the quantity andconcentration of the residual moisture can be kept in the red mud, thelower the losses. A limited quantity of washing water can be used forrecycling and/or washing out of these valuable substances. Any excess ofwater used, however, requires additional evaporation and evaporationcapacity.

Current processes particularly rely on counter current decantation(CCD). Settlers are used for separation, thickening and washing out ofred mud. The subsequent counter current decantation (CCD) consists of aseries of washing stages. Usually, 5 to 7 stages, each one with 2 ormore washers of at least 30 meters diameter, are used for the usualalumina capacities in the range of 1 million tons per year. Usually,these procedures require 3 to 5 m³ of water per ton of dry mud,exceeding the operational demand by 1 to 4 m³ per ton. Subsequently, thered mud is pumped into basins or lagoons, which are sometimes severalmiles away from the plant. Without any further treatment, the mud willfinally dry to a solids concentration of 50%.

Frequently, dram filters for red mud separation by filtration are usedas an alternative or supplement to counter current decantation. Vacuumfiltration is performed after 3 to 4 washing stages, permitting aresidual moisture of 45% with a washing water requirement of 2 to 3 m³per ton of dried mud. In this process too, pumps, particularlyhigh-pressure pumps for thick sludge, are used to pump the mud to adisposal pond where a final solids concentration after drying of up to65% can be reached.

These technologies for the removal of red mud have a number ofdisadvantages or inconveniences. Sodalite, a sodium aluminum silicate,leaves the circuit in a solid phase with the red mud, constituting agreat loss of chemicals, i. e. of caustic and alumina, which has a majoreffect on the production costs. Furthermore, plant efficiency isaffected by separation of red mud in relatively large equipment and theresulting elevated holding time. Temperature and caustic concentrationare reduced which leads to a reduction of liquor stability, resulting ina considerable and undesired loss of dissolved alumina; this, in turn,affects plant productivity and consequently the overall costs. This lossof soluble alumina may affect the supersaturation of process streamsand, in consequence, the liquor and plant productivity, thus increasingthe specific fixed costs. The final solids concentration of the red mudto be disposed of is still insufficient with 0.8 m³ to 1.5 m³ of liquorper ton of dry mud being wasted. Although the suspension has beendiluted by the washing water in counter current, considerable quantitiesof soluble caustic and alumina are disposed of as waste into the red mudponds. The excess water used for washing requires additional steam andevaporating capacity in order to maintain temperature and concentrationin the circuit, thus increasing production and capital costs. At asolids concentration of 30 to 55%, the red mud remains in a thixotropicrange with a large quantity of free caustic, which poses serioushandling and storage problems.

None of the known technologies has been able to solve the problem ofsolid phase losses resulting from the reactive silica content inbauxite.

SUMMARY OF THE INVENTION

The purpose of the present invention is to avoid the above-mentioneddisadvantages and inconveniences; it is particularly intended to reducethe solid phase (bound-form) losses of caustic and alumina, and toproduce non-thixotropic mud for clean and economical disposal. Whithinthe scope of the invention described herein, this problem is solved bywashing and filtration under pressure above atmospheric. Washing can bedone with hot water and/or steam. During pressurized washing andfiltration, particularly by means of hot water and/or steam, thevaluable substances bound in the solid (mud/waste) are dissolved andwashed out. This results in particular in a low caustic soda lyeconsumption, thanks to increased liquor recovery and/or regeneration,and--depending on the operating conditions--in a lower alumina loss. Atthe same time, the ecological compatibility of the red mud to bedisposed of is improved by reducing the quantity of liquid and causticcontained in it, and by removing thixotropic mud characteristics.

Another advantageous feature of the invention is characterized bywashing and filtration taking place at pressures between 2 and 8 bars,in particular at 4 to 5 bars. Working under high pressure permits use ofhot water at temperatures far above 100° C. which has a favorable effecton the washing process.

An advantageous feature of the invention is characterized by washingtaking place with thin red mud layers between 1 mm and 10 mm, preferablybetween 3 mm and 6 mm. This permits particularly efficient washing ofthe entire quantity of red mud.

An advantageous further embodiment of the invention is characterized bythe filter speed ranging between 0.1 and 3 revolutions per minute,preferably between 1 and 2 revolutions per minute.

A particularly advantageous feature of the invention is characterized byfiltration of the red mud to a residual moisture of 15 to 35 weight %,in particular 20 to 25 weight %. In this range, the resulting filtercake does not show thixotropic behavior any more, i.e. no furtherseparation of solids and liquid (acid), for instance during transportand/or storage, is to be expected.

Another advantageous feature of the invention is characterized by thewashing of the red mud being done with a water quantity above 0.5 m³,preferably above 1.0 m³ per ton of dry mud.

In addition, the invention covers a plant for dehydration/filtration andwashing of red mud by means of a filter, in particular for carrying outthe process. It is characterized particularly by the fact that thefilter is used in a pressure chamber.

An advantageous further development of the invention is characterized bya rotary filter, i.e. a disk filter, being used for filtration.

An advantageous embodiment of the invention is characterized by thepressure chamber being equipped with an additional steam cone above thefilter.

BRIEF DESCRIPTION OF THE DRAWINGS

An advantageous feature of the invention is characterized by adistribution system being provided which ensures that washing water isdistributed over the filter surface; when using a disk filter, thedistribution system can be placed on both sides of the filter disk(s)covering the major part of the filter surface in radial direction.

In the following, the invention is described on the basis of thedrawings.

FIG. 1a is the flow chart of one version of the Bayer process,

FIG. 1b shows the equipment diagram, of this version of the Bayerprocess as implemented,

FIG. 2 shows a typical version of state-of-the-art counter currentdecantation,

FIG. 3 shows a version according to the invention.

FIG. 4 shows one plant version and

FIG. 5 another plant version according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows a flow diagram of aluminum oxide (alumina) production frombauxite according to the Bayer process. In settler 1, the red mud isseparated from the process, passes a series of washing settlers 2(according to technical and economic requirements) onto filter 3. Duringthe filtration process, water 4 is added for washing out and,subsequently, the red mud 5 is pumped to a dump or into a sedimentationpond.

FIG. 1b shows the equipment diagram corresponding to FIG. 1a in which,parting from settler 1, the red mud is passed onto a counter currentwashing system. After being properly washed in a varying number ofwashing stages (usually 3 to 4), the red mud is filtered by means ofvacuum drum filters 3, washed, and subsequently the thickened red mud 5is pumped onto the dump.

In FIGS. 1a and 1b, the process flow described below and improved by thepresent invention is shown by a broken-line frame marked X.

FIG. 2 shows a typical standard solution for counter current decantationwith 5 washing stages. The quantities and concentrations of theindividual substance flows have been listed in table 1; these areapproximate values at a theoretical efficiency of 100%. Thepre-thickened red mud suspension is passed to the first washing settler10 through pipe 11. The settler underflow is subsequently passed ontothe next washing settler 20. The overflow 23 of washing settler 20 isadded to the feed 11 upstream of the entry to washing stage 10 forfurther washing out. The overflow 13 of the washing settler 10 issubsequently returned to the circuit. The further washing stages are ofanalogous design. The, underflow 52 of the washing settler 50 isdischarged and pumped onto a dump or a sedimentation pond 6. Afteranother settling process, a low quantity of caustic soda lye 7 isrecycled from the sedimentation pond 6, topped up with fresh water 8 andpassed onto washing stage 50 for washing out via a pipe 9. The red mudin the sedimentation pond has now a solids concentration ofapproximately 55% and a residual caustic concentration of approximately1.3%, the latter constituting a loss to the circuit since expensivefresh caustic must be added to the circuit elsewhere.

FIG. 3 shows the process according to the invention. In analogy to FIG.2, there are several washing stages. However, in this process only 4stages are needed. The underflow 42 of the washing settler 40 is passedonto a so-called pressure filter 60. For washing out, in addition, hotwater 8 is fed to this pressure filter 60. As an alternative, steam canbe fed to the filter. The filter cake 62 of the pressure filter 60contains approximately 75% solids and only approximately 1% of causticsoda lye which is lost and discharged into a sedimentation pond 6. Thepressure filter filtrate 63 is mixed with fresh water 8 and fed onto tothe fourth washing stage 40 for washing out via a pipe 9. This processpermits to increase the solids concentration from approximately 55% toapproximately 75% and to reduce the caustic soda lye loss toapproximately one tenth of the prior loss. The caustic soda lye washedout additionally is returned to the circuit in the filtrate 63 and viathe washing stages 10, 20, 30, 40.

FIG. 4 shows details of the part of the plant that concerns the pressurefilter. The pressure filter 60 consists of a pressure vessel 70, with afilter composed of a filter trough 71, filter disks 72, a motor 73, anda control head 74 located within the pressure chamber of the pressurevessel 70. In alternative versions, several filter disks mounted ontothe shaft or a filter drum can be used instead of the single filterdisk.

The underflow 42 of the washer 40 is passed into the tough 71 of thepressure filter 60 by means of a pump 64 where continuous cake formationtakes place and subsequently, during the dehumidification phase, thefilter cake formed from the suspension is washed. For this purpose,fresh water 8 is conveyed by means of a pump 78 and fed to the filterdisks or the filter drum through a suitable distribution system 83. Thefiltrate is passed via a control head 74 into a filtrate separator 75where the exhaust air 76 is separated from the filtrate enriched withcaustic soda lye 63. In this process, the filtrate from the cakeformation and the filtrate from the washing and/or dehumidification zonecan be removed separately. The solids are removed from the filter andpassed onto a dump 6 through a discharge lock 77 by means of suitableconveying equipment 65. Depending on the residual moisture of the filtercake, conventional conveyor systems such as conveyor belts or a pump forthick sludge can be used. The operating pressure required in thepressure vessel 70 for cake formation and dehumidification is producedby introducing air into the pressure vessel 70 by means of a compressor79. Furthermore, a small quantity of air at a slightly higher pressureis passed into the control head 74 and from there into the filter disk72 or the filter drum in order to separate and drop the filter cake fromthe filter medium.

FIG. 5 shows a similar plant in which steam is used instead of hot waterfor washing out the caustic soda lye from the red mud. For this purpose,a steam cone 82 is installed above the filter disk 72 and above thefilter trough 71. Steam is introduced into the cone through pipe 81.

The drawings show the features of the invention only by way of example.It would be a conceivable alternative, e.g., to produce the pressure inthe pressure vessel 70 by pure steam which would make it possible todispense with the compressed air feed and the pressure cone. Pressurefiltration without washing stages, or with two stages only, or withaddition of sand are further alternative applications. The use of a beltfilter in the pressure vessel is another possible alternative.

                  TABLE 1                                                         ______________________________________                                        Standard CCD                                                                                        Caustic conc.                                                                             Solids conc.                                Position                                                                             Location       g Na20/l    g/l                                         ______________________________________                                        10     Washer I       90          400                                         11     Feed           130         450                                         12     Underflow washer I                                                                           90          400                                         13     Overflow washer I                                                                            90          0                                           20     Washer II      60          400                                         21     Feed washer II 90          400                                         22     Underflow washer II                                                                          60          400                                         23     Overflow washer II                                                                           60          0                                           30     Washer III     40          400                                         31     Feed washer III                                                                              60          400                                         32     Underflow washer III                                                                         40          400                                         33     Overflow washer III                                                                          40          0                                           40     Washer IV      20          400                                         41     Feed washer IV 40          400                                         42     Underflow washer IV                                                                          20          400                                         43     Overflow washer IV                                                                           20          0                                           50     Washer V       10          400                                         51     Feed washer V  20          400                                         52     Underflow washer V                                                                           10          400                                         53     Overflow washer V                                                                            10          0                                           6      Sedimentation pond                                                                           16          900                                         7      Filtrate return from                                                                         10          0                                                  sedimentation pond                                                     8      Fresh water    0           0                                           9      Washing water feed                                                                           3.6         0                                           ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Pressure Filter tip                                                                                 Caustic conc.                                                                             Solids conc.                                Position                                                                             Location       g Na20/l    g/l                                         ______________________________________                                        10     Washer I       70          400                                         11     Feed           130         450                                         12     Underflow washer I                                                                           70          400                                         13     Overflow washer I                                                                            70          0                                           20     Washer II      40          400                                         21     Feed washer II 70          400                                         22     Underflow washer II                                                                          40          400                                         23     Overflow washer II                                                                           40          0                                           30     Washer III     25          400                                         31     Feed washer III                                                                              40          400                                         32     Underflow washer III                                                                         25          400                                         33     Overflow washer III                                                                          25          0                                           40     Washer IV      10          400                                         41     Feed washer IV 25          400                                         42     Underflow washer IV                                                                          10          400                                         60     Pressure filter                                                                              9           400                                         62     Filter cake    3           1500                                        63     Filtrate       9           0                                           8      Fresh water    0           0                                           8      Fresh water    0           0                                                  Pressure filter                                                        9      Washing water feed                                                                           3           0                                                  Washer IV                                                              ______________________________________                                    

We claim:
 1. A process for treating and disposing of red mud generatedin a Bayer process, said process comprising the steps of:feeding saidred mud to a filter trough enclosed within a pressure vessel andsubjecting said red mud to a pressure above atmospheric pressure whilein said trough, passing a filter surface through said red mud in saidfilter trough while under said pressure to recover a filter cake of redmud on said filter surface, directing a stream of washing fluid throughsaid filter cake on said filter surface while under said pressure towash caustic soda and alumina compounds from said filter cake and toproduce a washed dewatered filter cake, recovering said washing fluidfrom said filter cake and filter surface, said washing fluid containingsolubilized caustic soda and alumina compounds, and discharging saidwashed filter cake from said filter surface and said pressure vessel. 2.The process of claim 1, wherein said washing fluid is hot water at atemperature above about 80° C.
 3. The process of claim 1, wherein saidwashing fluid is hot water at a temperature of at least about 100° C. 4.The process of claim 1, wherein said washing fluid is steam.
 5. Theprocess of claim 1, wherein said washing fluid is saturated steam. 6.The process of claim 1, wherein said pressure vessel is maintained at anoperating pressure of about 2 to about 8 bar.
 7. The process of claim 1,wherein said pressure vessel is maintained at an operating pressure ofabout 4 to 5 bar.
 8. The process of claim 1, wherein said filter cake isa thin layer of about 1 mm to 10 mm.
 9. The process of claim 1, whereinsaid filter cake is a layer of about 3 mm to about 6 mm.
 10. The processof claim 1, wherein said filter surface is a rotary filter, said processcomprising rotating said rotary filter at a speed of about 0.1 to 3 rpm.11. The process of claim 1, wherein said filter surface is a rotaryfilter and said process comprises rotating said rotary filter at a speedof about 1 to 2 rpm.
 12. The process of claim 1, comprising recoveringsaid washing fluid from said filter cake to produce a filter cake havinga moisture content of about 15% to 35% by weight based on the weight ofthe filter cake.
 13. The process of claim 1, comprising recovering saidwashing fluid from said filter cake to produce a filter cake having amoisture content of about 20% to about 25% by weight based on the weightof the filter cake.
 14. The process of claim 1, comprising washing saidfilter cake with said washing fluid in an amount of at least 0.5 m³ perton of dry red mud.
 15. The process of claim 1, comprising washing saidfilter cake with said washing fluid in an amount of at least about 1 m³per ton of dry red mud.
 16. The process of claim 1, wherein said filtertrough includes a dome enclosing said filter surface and containedwithin said pressure vessel, said process comprising injecting steaminto said dome to contact said filter cake.
 17. The process of claim 1,comprising directing said stream of washing fluid substantially across awidth of said filter surface and said filter cake.
 18. The process ofclaim 1, wherein said filter surface is a disk filter having a first andsecond side, said process comprising directing said washing fluid ontosaid first and second sides.